Motor thermal capacity used — How does the relay know when I've reached 100%?

Abstract

How does a motor thermal overload element know when the motor has used 100% of its thermal capacity? In this paper the authors explore the answer to this question as it relates to the proper selection of the thermal overload curve. We also explore the coordination relationship between the overload curve and upstream time-overcurrent protective devices to better understand the 3-dimensional nature of the thermal over-load curve. Several real-life examples are used to illustrate the issues, challenges and consequences of curve selection.